void main() {
  var employee = Person(); // 创建一个 Person 实例

  employee.firstName = 'Bob'; // 设置属性值
  print('employee.firstName: ${employee.firstName}');

  // 由于 employee 已经被声明为 Person 类型，此处无需类型检查
  employee.firstName = 'Bob';
  print('employee is Person: ${employee.firstName}');
}

class Person {
  String? firstName;
}